FIG. 1 shows an example of a conventional ignition apparatus of the CDI type. This apparatus has a blocking oscillator 12. The blocking oscillator 12 comprises: a transformer 13 having four windings 13a to 13d; a transistor 14; diodes 15 and 16; resistors 17 and 18; and capacitors 19 and 20. An output voltage of a battery 11 is supplied to the base of the transistor 14 through the diode 15 and resistor 17, and the transistor 14 is turned on. Since a collector current flows through the winding 13a of the transformer 13 due to the turning-on of the transistor 14, a voltage is induced in the winding 13b in accordance with the voltage which is applied to the winding 13a. Since this voltage is applied to the base of the transistor 14 through the resistor 18, the base voltage decreases and the transistor 14 is turned off. Since no current flows through the winding 13a by the turning-off of the transistor 14, the voltage across the terminals of the winding 13a rapidly increases for only a moment and is stepped up by the transformer 13, thereby inducing pulse voltages in the windings 13b to 13d. Although the transistor 14 is reversely biased by the negative voltage pulse generated in the winding 13b, the base voltage of the transistor 14 gradually increases, so that the transistor 14 is again turned on. By repeating the foregoing operations, a pulse voltage is generated every predetermined period.
The pulse voltages developed in the windings 13c and 13d of the transformer 13 are supplied to an ignition circuit 21. The ignition circuit 21 comprises: transistors 22 and 23; diodes 24 to 28; thyristors (SCRs) 29 and 30; resistors 31 to 36; and capacitors 37, 39 and 40. One end of the winding 13c is connected to one end of a primary winding 41a of an ignition coil 41 through the diode 24 in the forward direction and through capacitor 37. A secondary winding 41b of the ignition coil 41 is connected to an ignition plug 42. the SCRs 29 and 30 and the diode 27 are forwardly serially connected between a connecting line of the diode 24 and capacitor 37 and the ground. On the other hand, a pulser coil 43 serves as an output coil of a pulse generator for generating a, sinewave pulse signal as an ignition timing signal for only one period at a predetermined angle position of a crank shaft. An output signal of the pulser coil 43 is half-wave rectified by the diode 28 and supplied to the base of the transistor 23 through the resistor 32. The transistors 22 and 23 are turned on each time an angle of crank shaft has reached a predetermined angle. An output voltage of the winding 13d is supplied to the gate of the SCR 29 through a trigger input circuit consisting of the diode 26, transistor 22, resistors 33 and 34, and capacitor 39. The output voltage of the winding 13d is also supplied to the gate of the SCR 30 through a trigger input circuit consisting of the resistors 35 and 36 and capacitor 40. Therefore, both SCRs 29 and 30 are turned on and all of the charges stored in the capacitor 37 are discharged to the primary winding 41a of the ignition coil 41, so that a high voltage is generated in the secondary winding 41b of the ignition coil 41 by the discharge energy. A spark discharge is generated from the ignition plug 42.
A stop control circuit 38 to stop the oscillating operation of the blocking generator 12 comprises: resistors 44 to 47; a transistor 48; a diode 49; a Zener diode 50; and a capacitor 51. When the SCRs 29 and 30 are turned on, gate voltages are supplied to the base of the transistor 48 through the resistor 45, so that the transistor 48 is turned on. The transistor 14 of the blocking generator 12 is turned off by the turning-on of the transistor 48, so that the oscillating operation of the blocking generator 12 is stopped. This stop operation is performed to prevent an overload being applied to the transformer 13 when the SCRs 29 and 30 are turned on. On the other hand, even if the withstanding voltage in the forward direction of the SCR decreases and a striking voltage of arc enough to turn on the SCR is not applied to the gate when the voltage between the anode and cathode of the SCR increases, the SCR is turned on. This situation is called a V.sub.BO ignition. To prevent such a V.sub.BO ignition, the SCRs 29 and 30 are serially connected as shown in FIG. 1.
The voltage generated in the winding 13c of the transformer 13 is supplied to the Zener diode 50 through the diode 49. Therefore, when this voltage exceeds the Zener voltage of the Zener diode 50, a base current flows through the transistor 48 and the transistor 48 is turned on. Thus, the oscillating operation of the blocking generator 12 is stopped in a manner similar to the above. This stop operation is executed to prevent the terminal voltage across the capacitor 37 from rising too high due to the charging.
However, in such a conventional ignition apparatus, even when the engine stops rotating, a pulse voltage is generated by the blocking oscillator and the capacitor is charged. Therefore, if the switching devices of the SCRs and the like have deteriorated even when an ignition timing signal is not supplied, a vain spark discharge is performed from the ignition plug by an increase of charging voltage of the capacitor in excess of a predetermined value.
On the other hand, in the case of a two-cycle engine, there is a case where a counter torque is generated depending on the positions of the piston and crank shaft at the time of ignition. Therefore, it is necessary to certainly block the generation of the counter torque.